youtubeVideoIdFrom

Remarks

A VideoLesson.source is an opaque URL: the project hosts most lectures as MP4 files, but some are published on YouTube. This is the one place that tells the two apart, so the player can pick a provider without a vendor conditional spreading through the component tree.

Returns undefined, not null, and never throws. A source that is not a YouTube link is the ordinary case — most lessons — not a failure, so there is no error to model. The codebase's Result<T, DomainError> convention governs use cases; this is a pure classifier at the delivery edge, and a discriminated union here would be ceremony with no caller for its error arm.

Parsed with URL, not matched with a regex. The host has to be matched exactly: https://cdn.example.com/youtube.com/watch?v=x.mp4 is a self-hosted file whose path mentions YouTube, and any pattern that scans the whole string for youtube.com would hand it to the wrong provider. Parsing also yields searchParams and a normalized pathname for free. A malformed source makes the constructor throw; that is caught and reported as "not YouTube", so bad data degrades to the direct-source path instead of breaking the lesson page.

Extra query parameters (si, list, t) are discarded. The provider owns the embed URL it builds, and the starting position is the stored playback position applied by a seek — not something encoded in a link.